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"The spread of nuclear weapons is an important issue in the theory and practice of international relations. The most fundamental reason why scholars and analysts study nuclear proliferation is that the spread of nuclear weapons may increase the likelihood of nuclear war. Although there has been a vigorous debate between nuclear optimists and nuclear pessimists over whether nuclear proliferation increases the risk of war, most analysts and policymakers have worried that war—including nuclear war—will become more likely as more states go nuclear.

There are many reasons to think that the spread of nuclear weapons will make it more likely that such weapons will be used. If more states have nuclear weapons, the probability that one leader will decide to use them may increase. Even if rational decisionmakers are likely to be deterred by the threat of nuclear retaliation, the possibility of inadvertent or accidental use remains.

The spread of nuclear weapons also increases the probability of theft of nuclear materials. Even if nuclear weapons have a stabilizing effect on relations between states, terrorist groups may be able to steal nuclear materials or nuclear bombs and then detonate a nuclear weapon in a major city, killing hundreds of thousands. Such concerns existed before the terrorist attacks of September 11, 2001, but have become more acute since. As more states acquire nuclear weapons, there will probably be more opportunities for theft by terrorists.

A state's quest for nuclear weapons can be a source of conflict even when such efforts are terminated, unsuccessful, or incomplete. A state that fears that an adversary is developing nuclear weapons may launch preventive attacks against its adversary's nuclear facilities. Israel bombed the nuclear reactor at Osirak, Iraq, in 1981 and attacked an apparent nuclear facility in Syria in 2007. The United States made plans to strike North Korean nuclear facilities in 1994. Many analysts believe that Israel or the United States might attack sites related to Iran's nuclear program. In extreme cases, a state may respond to concern over the nuclear program of a hostile state by launching a full-scale war. The 2003 U.S. invasion of Iraq was at least partially motivated by U.S. fears that Iraq was developing nuclear weapons.

Concern over nuclear proliferation is likely to increase in the coming years. Many observers believe that the spread of nuclear weapons to one or two more states will trigger a wave of new nuclear states. More states may turn to nuclear power to meet their energy needs as other sources of energy become more costly or undesirable because they emit carbon that contributes to global climate change. As more nuclear reactors are built, the world's stock of nuclear expertise and fissionable materials is likely to grow...."